Engineering of binder-free cobalt carbonate hydroxide hydrate nanostructure for high-performance hybrid supercapacitors

Cobalt-based binder-free electrode materials, such as Co(CO₃)_0.5(OH)⋅0.11H₂O nanowires (Co@Urea) and Co(OH)F microcrystals (Co@NH₄F), were synthesized on a nickel foam substrate by a traditional hydrothermal method using urea and ammonium fluoride (NH₄F) as the complexing reagents. The nanowire structure of the Co@Urea electrode promoted efficient charge transfer and high electrochemical active centers. The Co@Urea electrode exhibited a large specific capacity (693.0 C g⁻¹ at 1 A g⁻¹), better rate performance (50.8 % after 20 A g⁻¹), and exceptional cyclic durability (84.0 % capacity retention after 10,000 cycles) compared to Co@NH₄F electrode in a three-electrode configuration. A hybrid supercapacitor device was fabricated with Co@Urea as the cathode and activated charcoal as the anode, exhibiting high specific energy and specific power of 53.8 Wh kg⁻¹ and 799.9 W kg⁻¹, respectively. Furthermore, this hybrid device shows excellent long-term stability with 88.2 % capacity retention after 10,000 cycles at a current density of 40 A g⁻¹. This study proposes developing binder-free cobalt-based compounds for electrochemical energy storage applications.